1. Field of the Invention
The present invention relates to an optical element that includes a medium having a refractive index distribution (refractive index profile).
2. Description of the Related Art
An optical system used for an optical apparatus, such as a digital camera and a video camera, is demanded for a high performance and a small and lightweight configuration. In general, as the optical system becomes smaller, large amounts of a variety of aberrations occur, in particular a large amount of a chromatic aberration occurs, which is represented by an axial chromatic aberration and a lateral chromatic aberration, and its optical performance lowers.
It is difficult for an optical system made of an only optical material, such as known glass, to simultaneously satisfy the high performance and the small and lightweight configuration. Accordingly, one known method corrects (reduces) a variety of aberrations utilizing an optical element whose medium having a refractive index distribution for a part of the optical system.
This optical element having the refractive index distribution provides a higher degree of freedom of an aberrational correction than that of an optical element whose medium has a homogeneous refractive index. Each of Japanese Patent Laid-Open Nos. 2004-212644 and 2004-240464 discloses an optical system that includes an optical element having a refractive index distribution and used to correct a chromatic aberration.
Japanese Patent No. 3,573,575 discloses an optical system that includes a lens having a refractive index distribution in a direction orthogonal to the optical axis and its secondary spectrum is corrected while the dispersion is considered.
A material's characteristic in a shortwave region in the visible light is considered in the optical systems disclosed in Japanese Patent Laid-Open Nos. 2004-212644 and 2004-240464 and Japanese Patent No. 3,573,575, but a wavelength dispersion characteristic of a refraction caused by the refractive index distribution is not fully considered. Assume that an optical element having a refractive index distribution is equivalent to a dioptric optical element having no refractive index distribution, and values of its focal length, Abbe number, and partial distribution ratio are equivalently defined. The wavelength distribution of the refraction caused by the refractive index distribution in this case can be expressed by an equivalently defined Abbe number (which will be referred to as an “equivalent Abbe number” in this specification) and an equivalently defined partial distribution ratio (which will be referred to as an “equivalent partial dispersion ratio” in this specification), etc.
In particular, the secondary spectrum can be well corrected when the distribution characteristic of the refraction caused by the refractive index distribution for rays of four wavelengths including g line (435.8 nm), an F line (486.1 nm), d line (587.6 nm), and C line (656.3 nm).
In addition, Japanese Patent Laid-Open Nos. 2004-212644 and 2004-240464 and Japanese Patent No. 3573575 provide insufficient disclosures about a characteristic of a material etc. used to produce an optical element having a refractive index distribution, or do not clearly disclose a method for producing a target refractive index distribution.